The movement of pigment granules in chromatophores is a type of intracellular motility not dissimilar to that displayed by other cytoplasmic granules and vesicles and by chromosomes on the mitotic spindle. That all of these translocations depend on the integrity of microtubules for direction if not for motive force has been made abundantly evident in chromatophores as well as other systems. It will be the purpose of the proposed studies to explore the structural changes in the cytoplasmic ground substance associated with pigment motion in erythrophores of the red squirrel fish, Holocentrus ascensionis, Osbeck. Observations will be made by high voltage electron microscopy on whole erythrophores isolated from scales and maintained in culture. Preliminary studies have demonstrated that the pigment is supported (contained) within a fine 3-dimensional lattice of microtrabeculae and that this support changes its dimensions and form with each aggregation and dispersion of the pigment. The lattice appears to be guided in its "contraction" and restructuring (or "expansion") by a population of microtubules that persists in the cell cortex. To understand better the role of the lattice trabeculae in pigment motion it is proposeed (a) to increase observations on its morphology in the dispersed and aggregated states; (b) to observe in greater detail the sequential changes associated with pegment aggregation and dispersion; and (c) to explore the effects of a number of factors that can be added to the cell's environment such as specific ions (i.g. K ion and Ca ions), colchicine and dbcAMP.